Practical application of optical disk reproducing devices for reproducing optical disks such as a CD (compact disk) and an LD (laser disk) has been promoted in a wide range. Such optical disk reproducing devices are for reproducing information recorded in optical disks by converging a light beam such as a semiconductor laser emitted from a light source by the use of an objective lens so that the light beam is converged and projected on a signal surface of the disk, and detecting a reflected light from the optical disk by the use of a photodetector.
Recently further densification of the recording density is attempted with respect to the optical disks, which results in that disks differing from the conventional optical disks in standard exist.
In other words, a size of a pit which is a unit of recorded information is about 0.83 .mu.m in a CD, whereas it is 0.4 .mu.m in a DVD (digital video disk) which has been newly proposed. In addition, a space between rows of the information recording units, that is, a track pitch, is 1.6 .mu.m in the CD, whereas it is as narrow as 0.74 .mu.m in the DVD.
Incidentally, a recording density of an optical disk is determined in accordance with a size of a light spot of a recording-reproducing-use light beam emitted from an optical head (pickup) for reading a fine pit provided on the optical disk for information recording.
A diameter of the light spot is determined by a wavelength of the light beam used and a numerical aperture NA of the objective lens, and it is expressed as: EQU spot diameter=k.times.light beam's wavelength/NA
where k is a constant. Therefore, to make the light spot size smaller so as to read a high-density optical disk, it is necessary to use a light beam having a short wavelength or to use an objective lens having a greater numerical aperture NA.
Usually an optical head has only one objective lens, an optical head having an objective lens with respect to a high-density optical disk, that is, an optical lens having a great numerical aperture NA, usually cannot read a conventional-standard optical disk. This is because according to the conventional standard optical disk is allowed to have warp to some extent, whereas a high-density optical disk is only allowed to have slight warp because a light spot of an optical head for a high-density optical disk is easily distorted by any tilt of an optical disk with respect to an objective lens.
Note that distortion of the light spot is affected by thickness of the optical disk. That is, if the optical disk is thin, the light spot distortion is small even when the disk is tilted. Therefore, a thin disk is sometimes used as a substrate of a high-density optical disk.
Incidentally, various optical recording and reproducing devices which are, even when used alone, capable of reproducing various types of optical disks differing in specification and standard are proposed now.
For example, the Japanese Publication for Laid-Open Patent Application No. 6-124477/1994 (Tokukaihei No. 6-124477) discloses an optical head arranged as follows: a liquid crystal filter is provided between a collimating lens and a deflecting beam splitter, and by adjusting a voltage applied to electrodes provided in the liquid crystal filter, a diameter of a light flux entering an objective lens is changed, thereby switching a numerical aperture NA of an objective lens. Therefore, a reproducing device having the optical head thus arranged is capable of, even when used alone, reproducing optical disks of different recording densities.
The Japanese Publication for Laid-Open Patent Application No. 5-54406/1993 (Tokukaihei No. 5-54406) discloses an optical head arranged as follows: a spherical aberration compensating lens for converging a light beam without aberration, which corresponds to a plurality of disk substrates with different thicknesses, is inserted in and taken away from a light flux entrance side of an objective lens. By doing so, a reproducing device having this optical head is made capable of, even when used alone, reproducing various optical disks differing in thickness.
However, with any of these conventional techniques, it is impossible to freely set a position of a focus of the objective lens, and hence a motion range of the objective lens is naturally determined in accordance with a thickness of the optical disk and deflection (caused by warp and others of the disk) which is regulated by the standard. The thickness dimension of the optical head is determined in accordance with the motion range of the objective lens. Herein, position relations between a DVD 11 and a objective lens 4 and between a CD 12 and the objective lens 4 are shown in FIG. 16. Note that a left half from a center line illustrates a state of a reproducing operation with respect to the DVD 11, whereas a right half from the center line illustrates a state of a reproducing operation with respect to the CD 12.
As shown in the figure, in the case where an optical head corresponding to the DVD 11 is used for reproducing the CD 12, if only a working distance 2 of the CD 12 (a distance from the objective lens 4 to the CD 12; hereinafter referred to as WD2) is set, a working distance 1 of the DVD 11 (a distance from the objective lens 4 to the DVD 11; hereinafter referred to as WD1) is automatically determined in accordance with a difference between positions of information recording surfaces of the DVD 11 and the CD 12, which is, a difference between respective optical distances of passage of light through the disks, that is, respective distances from lower surfaces of the disks to the information recording surfaces (a difference between the substrate thicknesses Dt1 and Dt2).
Let the substrate thickness Dt1 of the DVD 11 be 0.6 mm, and the substrate thickness Dt2 of the CD 12 be 1.2 mm, and a difference (physical difference) between positions of the spot on the DVD 11 and the CD 12 is found as Dt2-Dt1=0.6 mm. Then, the optical distance (equivalent to WD1-WD2), found by dividing the physical distance by an index of refraction, is about 0.6/1.5=0.4.
Therefore, since the CD 12 has WD2 of about 1.6 mm (described later), WD1 of the DVD 11 is found as follows: ##EQU1## Therefore, in the conventional arrangement, the working distance WD1 of the DVD 11 is too great, and as a result there arises a problem that the optical head becomes bulky.